U.S. hostage Mueller had her fingernails pulled out before being repeatedly raped by ISIS leader

A Yazidi woman kidnapped and detained as Abu Bakr al-Baghdadi's personal sex slave has told how the ISIS leader repeatedly raped U.S. hostage Kayla Mueller – who he had taken as a secret bride.

Speaking to MailOnline, blue-eyed Muna, 16, revealed that after his depraved jihadi cohorts ripped out Miss Mueller's finger nails, Baghdadi told her that she would become his wife 'by force'.

But despite his image as the world's most notorious terrorist, Baghdadi was terrified that his other wives would find out about the 26-year-old aid worker, so insisted she was imprisoned at the home of his deputy Abu Sayaff.

Sayaff was allegedly killed alongside Miss Mueller, 26, in an airstrike on the property earlier this year.

Muna's harrowing account of her time as a sex slave gives unprecedented insight into the life, cruelty and daily routines of elusive terror chief Baghdadi, who has only made one public appearance since he declared himself caliph - leader of the world's Muslims - during a sermon in Mosul last summer.

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Revealed: Speaking to MailOnline, blue-eyed Muna revealed that after his depraved jihadi cohorts ripped out Miss Mueller's finger nails, Baghdadi informed her that he planned to take her as his wife 'by force'

Destroyed: Kayla Mueller was detained at the home of his deputy Abu Sayaff, who was allegedly killed alongside Miss Mueller in an airstrike on the property earlier this year (pictured)

Yazidi sex slave claims she was raped by Abu Bakr al-Baghdadi

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With a face veil covering everything but her piercing blue eyes, Muna revealed the horrors she witnessed while being moved from location to location to be abused at Baghdadi's convenience.

She told MailOnline how the terror leader picked her out from 61 Yazidi women and girls aged between nine and 22 who were among the hundreds taken hostage during last year's devastating siege of Mount Sinjar.

The average age of those 'sold at the slave market was 15', Muna said, adding that it took place at a Christian building in Mosul, which she described as a 'a white palace'.

Having been held for two nights under the guard of five militants at a property in ISIS' de facto capital Raqqa, Baghdadi himself turned up at around 10pm on August 15 last year.

He was accompanied by a 30-year-old Kurdish man named Mansour from the Iraqi city of Kirkuk, who is understood to be both his son-in-law and bodyguard.

Having been selected as his sex slave, Muna told MailOnline how Baghdadi transported her to a two-storey house elsewhere in the centre of Raqqa, where he lived with three other wives, three sons and three daughters - the eldest of which was Mansour's wife.

'The wives were always worse than Baghdadi,' Muna said. 'They were always telling the children that they were lazy and beating them.'

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After he failed escape Muna (left) was transported to a Raqqa jail, where she first met Miss Mueller (right)

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Victim: There was no formal wedding ceremony but after claiming American aid worker Kayla Mueller (pictured) as his wife, Baghdadi forced her to wear a hijab and spent his days teaching her about the Koran

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Eyewitness: Muna's harrowing account of her time as a sex slave gives unprecedented insight into the life and daily routines of elusive terror chief Baghdadi

Having taken Muna into his home, Baghdadi set about trying to convert her to Islam - beginning by showing her the sickening filmed execution of American hostage James Foley at the hands of British national Mohammed Emwazi, better known ISIS' depraved chief executioner Jihadi John.

'He said he ordered them to kill James Foley,' Muna revealed, adding that Baghdadi referred to her as an infidel who would only go to heaven if she converted to the 'nice and clean' religion of Islam.

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Prisoner: Kayla was captured by ISIS on August 3, 2013, while driving to southern Turkey after dropping off a medical worker in Aleppo

The implication was clear, Muna said. Convert to Islam or die a brutal death like James Foley.

While in Raqqa, Baghdadi attempted to brainwash his Yazidi sex slaves by telling them to forget their husbands, fathers and brothers as ISIS had killed them all.

Muna told MailOnline that after 10 days in Raqqa, she and the Baghdadi family were moved to the province of Aleppo, also known as Halab, where she attempted a daring nighttime escape.

Tragically she was swiftly recaptured by ISIS guards and returned to Baghdadi.

Muna was reluctant to speak of the abuse she personally suffered at the hands of the ISIS leader, but did say that he beat her all over her body with a belt and garden hoe, leaving her body covered in bruises.

Telling her she 'belongs to the Islamic State', the terror chief also slapped her in the face until her nose bled and beat her around the face with cables and wooden sticks.

Suffering pain from the attacks to this day, Muna said one beating from Baghdadi was so brutal that it left her with a dislocated shoulder and her friend with bone fractures in the face.

Shortly afterwards Muna was transported to a female prison in Raqqa, where she first met Miss Mueller.

The American aid worker was kidnapped by jihadis in August 2013 following a short visit to Syria from her base over the border in Turkey.

It is believed that during her brief visit to a Doctors Without Borders-run hospital in Aleppo, a spy alerted local Islamist militants to her presence and she and her Syrian photographer boyfriend Omar Alkhani were ambushed by gunmen shortly afterwards.

The wives were always worse than Baghdadi...They were always telling the children that they were lazy and beating them.

Mr Alkhani told his captors that Miss Mueller was his wife and pleaded for her release. The militants went to her to corroborate the story and told her she would be freed if she told the truth.

Tragically she did just that and admitted that she was unmarried. Shortly after the ISIS fighters began the process of finding her a husband, with Baghdadi eventually claiming her for himself.

Sunni Muslim Alkhani was later released by the terror group after being quizzed on his aid work.

'When they first captured Kayla, ISIS fighters pulled her nails out,' Muna told MailOnline. 'She was forced to convert to Islam,' she added.

Muna said she was transported alongside Miss Mueller and another Yazidi sex slave named Susan to the home of Baghdadi's deputy Abu Sayaff in the Al-Shadadiya suburb of Raqqa.

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Fear: Despite his image as the world's most notorious terrorist, Baghdadi (pictured) was terrified that his other wives would find out about the American aid worker

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Captor: Kayla was reportedly kept as Baghdadi's wife at the Syrian home of best friend and deputy Abu Sayyaf (pictured) the man in charge of ISIS' oil and gas revenue

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Jihadis: Having been held for two nights under the guard of five militants at a property in ISIS' de facto capital Raqqa (pictured), Baghdadi himself turned up at around 10pm on August 15 last year

In October, Baghdadi himself visited the house and repeatedly raped Kayla in front of Muna.

'She was afraid and she didn't resist as he would beat her,' she said. 'I saw him rape her three or four times and they told us that anybody who runs away, including Kayla, would be killed...Kayla did not know she was the Caliph, but she knew he was important.'

'When Kayla returned in the morning she was crying and said she'd been raped,' Muna added.

Despite being the world's most wanted terrorist, Baghdadi kept his marriage to Miss Mueller a secret because he didn't want his other wives to find out.

'Umm Sayaff [Abu Sayaff's wife] told Kayla that Baghdadi's wives did not know he had been married to her.'

[Kayla] didn't resist. She said her friend James Foley had already been beheaded and she feared suffering the same fate.

There was no formal wedding ceremony but after claiming the American aid worker as his wife, he forced her to wear a hijab full body covering and spent his days teaching her about the Koran.

His jealousy was such that even the man whose house she was imprisoned in - Baghdadi's best friend and deputy Abu Sayaff - was banned from looking at her, Muna said.

'He respected Kayla as Baghdadi's wife. They never talked about the military airstrikes in front of them,' she added.

Giving unprecedented details about Baghdadi's day to day life in the vast swathes of land under his control, Muna told MailOnline that the ISIS leader and his family had a penchant for expensive watches and would eat takeaways every evening rather than cook or go out in public.

Despite the strict security conditions she was kept in, Muna managed to escape ISIS' clutches on October 8 last year,

'One night me and Susan told Kayla we were planning to runaway but she said the last time you guys tried to escape you were captured and beaten. This time they will kill you,' Muna said.

'Susan and I insisted on running away and told Kayla to come with us. But she said her friend [James Foley] had already been beheaded and she feared suffering the same fate,' she added.

Kayla Mueller expresses 'solidarity with the Syrian people'

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Despite being the world's most wanted terrorist, Baghdadi kept his marriage to Miss Mueller a secret because he didn't want his other wives to find out. Kayla is pictured above with her mother Marsha

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Imprisonment: Kayla pictured above with her father Carl, celebrating a new baby, in a picture posted to Facebook before her death

Incredibly Muna was able to reach a house three hours from her prison and from there was able to contact Kurdish forces who rescued her and reunited her with her brother who, despite Baghdadi's claims, hadn't been killed.

Now Muna plans to join the hundreds of thousands of migrants fleeing the Middle East for Europe, telling MailOnline that she plans to travel to Germany at some point in the next few weeks.

There are still a number of questions surrounding Miss Mueller's February 10 death, which ISIS claimed came as the result of a coalition airstrike - possibly involving Jordanian warplanes.

The Pentagon maintains that Miss Mueller was in fact murdered by her captors and U.S. special forces struck the Sayaff compound only after they learned that she had been killed.

Friends and family gather to honor Kayla Mueller in February

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The elite Delta Force team raided the Syrian compound in May, and killed Sayyaf while taking into custody his wife and a treasure trove of ISIS intelligence. Umm Sayyaf gave extensive information to U.S. interrogators and has been turned over to the Iraqi Kurds for trial.

The last time you guys tried to escape you were captured and beaten. This time they will kill you.

Kayla Mueller to Muna

Speaking of the moment she learned of her friend's death, Muna told MailOnline: 'I was so sad, but I don't believe she was killed in an airstrike.'

'I would tell her parents the whole story about Kayla. She was always saying that if she gets released she will take me to her home to meet them.

'I would tell them that she was such as close friend to me - more than a sister. I loved Kayla.'

Muna's harrowing story comes just one day after Hollywood star Angelina Jolie told the Houses of Parliament that ISIS is using rape as the 'centrepoint of their terror' and warned of the destruction caused by sexual violence in conflict zones.

The actress and human rights activist spoke of the harrowing moment she met girls as young as seven who had been brutally raped in war zones.

Muna's account proves that ISIS campaign of sexual abuse goes right to the top, with terror leader Abu Bakr al-Baghdadi owning sex slaves and regularly taking part in horrific rapes.

MailOnline understands from security sources that the American intelligence officials have spoken with Muna.